The present invention relates to fluoropolymers, and more particularly relates to cross-linkable aqueous fluoropolymer based dispersions which contain silanes. The present invention further relates to cross-linked products.
While there are many high performance fluorine-containing coatings, these coatings generally suffer from a variety of shortcomings. For instance, many fluoropolymer coatings are prepared from solutions or solvent-based dispersions of fluoropolymers and other resins. Many of these coatings, including most coatings based on polyvinylidene fluoride, require high temperature baking for optimal performance. Due to safety and other environmental reasons associated with solvents, water borne coatings in the form of an aqueous dispersion are preferred over solvent-based coatings. Coatings which do not require baking are particularly preferred since they can be used on a wider range of substrates and applied under various conditions. Generally, coating performance is often achieved by having an intimate blend of fluorinated and non-fluorinated polymers or by copolymerizing fluorinated and non-fluorinated monomers. However, for many desirable groups of co-monomers, copolymerization can be difficult if not impossible. Furthermore, producing an intimate homogeneous blend of fluorinated and non-fluorinated resins can be severely limited by polymer incompatibility, particularly when the fluorine content of the coating is high such as over 50% by weight. A lack of homogeneity in the mixing of the resins can greatly reduce the coating performance. Dispersions have been attempted based on chlorine containing monomers along with vinyl containing polymers in order to achieve homogeneous mixing of two polymers. However, the presence of chlorine reduces the level of fluorine in the polymer which adversely affects the weatherability of the resulting film due both to the strength of Cxe2x80x94F bonds and the weakness of Cxe2x80x94Cl bonds. The presence of chlorine also raises the refractive index of the polymer which is undesirable. Accordingly, a chlorine-free dispersion is highly desirable.
Furthermore, there is a desire to improve the low surface energy of fluoropolymers since the adhesion of coatings to various substrates is often inadequate. Also, there is a desire for water-based dispersions of fluoropolymer resins having good stability during storage and yet producing cross-linked films at room temperature with good clarity and solvent resistance upon application and drying.
Various other attempts to provide aqueous compositions are reported. For instance, U.S. Pat. No. 5,621,038 relates to the preparation of storage stable curable, aqueous compositions by incorporating sterically hindered silane groups into the polymer and using a hydrolytically stable organo metallic catalyst. An external buffer may be used to maintain the pH of the composition between 5.5 and 8.0 to reportedly prevent premature hydrolysis-condensation of the silane groups. However, this patent does not relate to fluoropolymers nor address the storage stability problems with fluoropolymers. U.S. Pat. No. 4,526,930 relates to silane-containing polymers. However, this patent is not directed to fluoropolymers and again does not address the storage stability problems associated with fluoropolymers. Furthermore, in this patent, an organosilane moiety is post-polymerized or grafted to a molten thermoplastic polymer in the presence of a peroxide indicator. The resulting polymer is not considered an aqueous dispersion. JP 05170909 relates to aqueous dispersions prepared by conducting an organosilane condensation reaction in the presence of a fluoropolymer aqueous dispersion. Hydrolytically unstable silanes can be used to create in situ polysiloxane polymers in the aqueous dispersion. EP 736 583 relates to aqueous dispersions containing organosilicon compounds admixed with an aqueous fluoropolymer dispersion. The admixture of the organosilicon compounds to the fluoropolymer dispersion is preferably done after the polymerizations to make the fluoropolymer dispersion. In addition, this patent does not address the need for storage stability with fluoropolymer dispersions to prevent hydrolysis/condensation of the silane functional groups. In U.S. Pat. No. 4,514,537, silane functional materials which are post-added to fluoropolymer dispersions are described. Again, like the patents described above, this patent does not address the need for storage stability by preventing hydrolysis/condensation of the silane functional groups. In unexamined JP 06322313, a silane-containing fluoropolymer dispersion is produced by blending two dispersions. One dispersion contains poly(fluoro-monomer-co-vinyl ether) and the other dispersion contains an acrylic polymer with a reactive organosilane group. This essentially is a post-addition of a silane functional polymer to a fluoropolymer. Again, this patent does not address or appreciate achieving storage stability by preventing hydrolysis/condensation of the silane functional groups. All of the patents and publication mentioned above and throughout this patent application form a part of this application and are incorporated herein in their entirety by reference.
Accordingly, there is a need to overcome one or more of the above-described shortcomings with respect to fluoropolymer dispersions.
Features of the present invention are to provide a storage stable aqueous fluoropolymer dispersion; a cross-linkable fluoropolymer dispersion which is a chlorine-free fluoropolymer; aqueous fluoropolymer dispersions having superior water borne film forming capability and which achieve cross-linking after film formation and which can be accelerated by the use of heat, catalysts, or a change in the pH; water-based paints and coatings using the stable cross-linkable fluoropolymer dispersions; solvent dispersions, powder coatings, or melt-process material derived from cross-linkable fluoropolymer dispersions which are storage stable; and methods to make stable fluoropolymer dispersions.
To achieve these and other advantages and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention relates to a cross-linkable fluoropolymer blend which contains a) at least one polymer containing acrylic units, vinyl units, or both, and at least one hydrolytically stable silane or silane containing group; and b) at least one thermoplastic fluoropolymer, wherein a) and b) are different; to a cross-linkable fluoropolymer dispersion containing a polymer product resulting from polymerizing at least one polymerizable acrylic and/or vinyl containing monomer and at least one hydrolytically stable silane monomer in the presence of an aqueous dispersion of at least one fluoropolymer; to cross-linked versions of the above-described fluoropolymer dispersions; to paints and coatings containing the cross-linkable or cross-linked fluoropolymer dispersions of the present invention; to solvent dispersions, powder coatings, or melt-processed material derived from the cross-linkable fluoropolymer dispersions of the present invention which can be obtained after removal of the water from the dispersion; to films or articles made from the aqueous fluoropolymer dispersions of the present invention which preferably have a low refractive index and/or have excellent UV and chemical resistance; and to methods of preparing cross-linkable fluoropolymer dispersions by polymerizing at least one polymerizable acrylic and/or vinyl containing monomer and at least one hydrolytically stable silane monomer in the presence of an aqueous dispersion of at least one fluoropolymer. Alternatively, or in addition, the cross-linkable fluoropolymer dispersion can be prepared by polymerizing at least one polymerizable acrylic and/or vinyl containing monomer in the presence of an aqueous dispersion of at least one fluoropolymer which contains organosilane groups as part of the fluoropolymer.